Optical scanning devices for deflecting a light beam are widely used in digital copying machines, laser printers, bar-code readers, scanners, and projectors, etc. Motor-actuated polygon mirrors and galvanomirrors have generally been finding use as such optical scanning devices.
In recent years, microprocessing technology has made great progress in the development of optical scanning devices based on MEMS (MicroElectroMechanical Systems). Particularly, attention has been paid to MEMS optical scanning devices which deflect a light beam by reciprocally vibrating a movable mirror about a beam used as a rotational shaft.
Movable mirrors fabricated according to the MEMS technology are advantageous in that they are structurally simpler, can be fabricated as an integral structure by the semiconductor fabrication process, can thus easily be reduced in size and cost, and hence can be operated at a higher speed, as compared with optical scanning devices with a motor-driven polygon mirror.
Optical scanning mirrors based on the MEMS technology are generally manufactured as a resonant mirror wherein the drive frequency and the resonant frequency of a structural body agree with each other for an increased deflection angle. The resonant frequency fr of the mirror is given from the modulus k of torsional elasticity of the beam and the moment lm of inertia of the mirror according to the following equation:fr=1/(2π)(k/lm)1/2  (1)
If the drive force applied to the mirror is represented by T, then the deflection angle θ of the mirror is given by the following equation:θ=QT/k  (2)where Q represents the quality coefficient of the system, which has a typical value of about 100 in air and a typical value of about 1000 in vacuum. The equation indicates that the mirror actuated in resonance can be deflected greatly even if the drive force is small.
For increasing the deflection angle θ of the mirror, it is necessary to increase the drive force T or the quality coefficient Q.
One of schemes for producing a drive force is to use a piezoelectric element. Only optical scanning devices which incorporate a piezoelectric actuator will be described below. Optical scanning devices include a perpendicular layout type and a parallel layout type which are different from each other depending on whether the longitudinal direction of the piezoelectric element and the longitudinal direction of the torsional beam are perpendicular to each other or parallel to each other.
Examples of the optical scanning device of the perpendicular layout type are disclosed in Patent documents 1 through 4. The optical scanning devices disclosed in Patent documents 1 through 4 have a plurality of boards joined to a torsional beam and a plurality of piezoelectric elements mounted respectively on the boards. The optical scanning device of this type are characterized in that voltages of independent phrases are applied to the respective piezoelectric elements to generate a drive force in a rotational direction about the torsional beam.
Examples of the optical scanning device of the parallel layout type are disclosed in Patent document 5 and Patent document 6. Each of Patent document 5 and Patent document 6 reveals an optical scanning device wherein a torsional beam joined to a mirror is connected to a bifurcated board and piezoelectric elements are mounted on the respective board arms. In the optical scanning device of this type, voltages having independent phases are applied to the respective piezoelectric elements to actuate the optical scanning mirror.
The optical scanning devices disclosed respectively in Patent document 5 and Patent document 6 are characterized in that the longitudinal directions of the piezoelectric elements mounted on the respective board arms for imparting a rotational force to the torsional beam are in agreement with the longitudinal direction of the torsional beam. According to Patent document 5, there is a limitation in which the branch width of a second spring should not exceed the width of the reflecting mirror. Patent document 5 reveals that the limitation is needed in order to prevent the torsional beam from generating a vibration mode in a flexural direction in a frequency range lower than the natural frequency of the torsional beam in its rotational direction.